Anaplasma phagocytophilum, interferon gamma production and Stat1 signaling

Abstract

Human granulocytic anaplasmosis (HGA) is caused by the obligate intracellular bacterium, Anaplasma phagocytophilum. The proinflammatory cytokine, IFN-γ is necessary for innate immunity and plays an important role in the induction of severe histopathology in A. phagocytophilum-infected mice and disease in horses and humans. In this study, we examined the role of activation of signal transducer and activator of transcription (Stat) 1 phosphorylation with A. phagocytophilum infection, and found it to be markedly increased at day 7 post infection compared to mock-infected controls. This increase in phosphorylated Stat1 (pStat1) was significantly correlated with IFN-γ production and inflammatory tissue injury. Since pStat1 operates as a transcription factor central to the generation of effectors of inflammatory injury, these data suggest that Stat1 signaling is involved in IFN-γ-mediated immunopathologic lesions and disease in A. phagocytophilum infection and could be an important target for intervention of disease.

Stat1 Negatively Regulates Immune-Mediated Injury with Anaplasma phagocytophilum Infection

“…We previously showed that phosphorylated Stat1 is not expressed or weakly expressed in splenocytes of uninfected mice, but peaks in splenocytes from infected mice at day 7 post A. phagocytophilum infection (17). To further determine whether Stat1 and phosphorylated Stat1 are required for the induction of iNOS, we examined iNOS protein expression at when pStat1 expression peaks on day 7 post A. phagocytophilum infection.…”

“…While IFNγ signaling is important for control of A. phagocytophilum infection in vivo , it is separately a major activator of inflammatory tissue injury in mice and this correlates with inflammatory histopathology that belies disease severity in horse models and in humans (5, 7, 17, 18). IFNγ and its signaling through phosphorylation, homodimerization and nuclear translocation of Stat1, leads to macrophage activation, and activated macrophages represent the dominant effector phase of the innate immune response and subsequent host tissue injury and disease (1, 17, 22). As a result, A. phagocytophilum infection in IFNγ KO mice results in absence of histopathologic tissue damage despite markedly increased bacterial loads (7).…”

“…In contrast, deficiency of IL-10 (IL-10 KO mice) that signals via Stat3, leads to restriction of microbial growth but marked enhancement of inflammatory tissue injury with extensive necrosis and pyroptosis, presumably owing to its role in counterbalancing IFNγ effects (7). Thus, since “disease” with A. phagocytophilum is immune-driven, and in spite of the role Stat1 plays in suppressing microbial growth in vivo , a critical question is whether features of disease severity could be suppressed by dampening or eliminating Stat1 signaling (17, 18). This would also necessarily impact signaling by type I and type III IFNs (IFNα, IFNβ and IFNλ) that generate interferon stimulated gene factor 3 complex (ISFG3 or Stat1/Stat2/interferon regulatory factor [IRF] 9) to bind IFN-stimulated response elements (ISRE) in genes and activate inflammatory antiviral responses as well (13, 15).…”

“…Bacterial loads in the spleen were determined in WT and Stat1 KO mice following A. phagocytophilum infection as described (17). Briefly, spleens were soaked in RNA later (Qiagen Inc., Valencia, CA) and stored at 4°C.…”

“…The loss of STAT1 function in humans or mice results in a dramatically increased susceptibility to viral, bacterial, and protozoan infections (13, 14, 16). A. phagocytophilum infection-induced IFNγ signaling leads to phosphorylation of Stat1 in mice (17), and dexamethasone suppression of STAT1 expression reduces inflammatory signaling and disease severity in A. phagocytophilum -infected horses (18). However, the direct effects and impact of STAT1-mediated signaling on host immunity during A. phagocytophilum infection has not been otherwise studied.…”